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<h2 id="变量、复制"><a href="#变量、复制" class="headerlink" title="变量、复制"></a>变量、复制</h2><h3 id="let-const"><a href="#let-const" class="headerlink" title="let&#x2F;const"></a>let&#x2F;const</h3><ul>
<li><p>let 相对 var</p>
<ul>
<li>没有变量提升，不能先使用后定义、</li>
<li>不能重复定义</li>
<li>存在块级作用域</li>
</ul>
</li>
<li><p>优先使用 const，<code>必须有初始化值</code>，除非值一定需要改变</p>
<ul>
<li>const 只是指向的对象不能修改，可以改<code>对象属性</code></li>
<li>相当于只能修改内容，不能修改内存地址</li>
</ul>
</li>
<li><p>let&#x2F;const 有 if 和 for 的块级作用域</p>
</li>
<li><p>作用域:变量在什么范围内是可用的</p>
<ul>
<li>块级作用域:{}、if(){}、for(){}…<ul>
<li>暂时性死区 TDZ(如果块中定义了 let 或 const，区块中对它们声明的变量形成封闭作用域，即使外面有声明同样的变量，快内申明之前用都报错)</li>
<li>即使有块级作用域，外面 let 过的变量，{}内部也不能重复定义,外面 var 的，就可以</li>
<li>同一个块内，无论 let 还是 var 都不能定义相同的变量</li>
</ul>
</li>
<li>es5 以前 var <code>if和for</code>都没有块级作用域，很多东西都需要<code>借助function</code>的作用域来解决问题</li>
<li>var for 循环的异常<ul>
<li>就是因为 var 没有块级作用域， 变量 i 一直被改变，不会保存在作用域中,事件操作用的时候 i 就是最后一次的值了</li>
<li><code>闭包</code>(function(i){})(i)可以解决问题是因为<code>函数是有作用域的</code>,传入的时候就形参赋值了</li>
</ul>
</li>
</ul>
</li>
</ul>
<h2 id="运算符"><a href="#运算符" class="headerlink" title="运算符"></a>运算符</h2><h3 id=""><a href="#" class="headerlink" title="??"></a>??</h3><blockquote>
<p>非空运算符，只判断变量是否为<code>null/undefined</code>, <code>||</code>可以判断变量是否为 null&#x2F;undefined&#x2F;false&#x2F;0&#x2F;“”等</p>
</blockquote>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//?? 如果前面变量为null或undefined则返回??后面的值</span></span><br><span class="line"><span class="literal">null</span>??<span class="number">1</span>	 <span class="comment">//1</span></span><br><span class="line"><span class="number">0</span>??<span class="number">1</span>	 <span class="comment">//0</span></span><br><span class="line"><span class="comment">// 非空判断</span></span><br><span class="line"><span class="keyword">if</span>((value??<span class="string">&#x27;&#x27;</span>) !== <span class="string">&#x27;&#x27;</span>)&#123;</span><br><span class="line">  <span class="comment">//...</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">//??=</span></span><br><span class="line">a ??= b -&gt; a = (a??b)</span><br><span class="line"></span><br><span class="line"><span class="comment">//?. 可选链操作符</span></span><br><span class="line"><span class="comment">// const host = config &amp;&amp; config.db &amp;&amp; config.db.host;</span></span><br><span class="line"><span class="comment">// 查找config下的db下的host属性，如果那一层没有，直接返回undefined</span></span><br><span class="line"><span class="keyword">const</span> host = config?.<span class="property">db</span>?.<span class="property">host</span>;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(host)</span><br><span class="line"></span><br><span class="line">obj?.<span class="property">prop</span> <span class="comment">// 对象属性是否存在</span></span><br><span class="line">obj?.[expr] <span class="comment">// 同上</span></span><br><span class="line">func?.(...args) <span class="comment">// 函数或对象方法是否存在</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//?: 三元运算符</span></span><br></pre></td></tr></table></figure>

<h3 id="-1"><a href="#-1" class="headerlink" title=""></a></h3><h2 id="函数"><a href="#函数" class="headerlink" title="函数"></a>函数</h2><p>函数新增<br>1、形参可以有默认值，<br>2、形参支持解构<br>3、剩余参数</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> user = &#123;</span><br><span class="line">  <span class="attr">name</span>: <span class="string">&quot;123&quot;</span>,</span><br><span class="line">  <span class="attr">age</span>: <span class="number">100</span>,</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="keyword">function</span> <span class="title function_">fn</span>(<span class="params">&#123; name, age &#125;, a, b = <span class="number">10</span>, ...c</span>) &#123;</span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(name, age); <span class="comment">// 123 100</span></span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(a); <span class="comment">// 1</span></span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(b); <span class="comment">// 2</span></span><br><span class="line">  <span class="variable language_">console</span>.<span class="title function_">log</span>(c); <span class="comment">// [3,4]</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="title function_">fn</span>(user, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>);</span><br></pre></td></tr></table></figure>

<h3 id="箭头函数"><a href="#箭头函数" class="headerlink" title="箭头函数"></a>箭头函数</h3><ul>
<li><code>const fun = a =&gt; 123</code></li>
<li><code>const fun = () =&gt; 123</code></li>
<li>一个参数可胜利(),只有返回值可以省略{}</li>
<li><code>this</code>:箭头函数 this 是最近作用域的 this，向外层作用域一层层查找</li>
</ul>
<h2 id="数组-json"><a href="#数组-json" class="headerlink" title="数组&#x2F;json"></a>数组&#x2F;json</h2><ul>
<li>数组<ul>
<li>数组遍历<ul>
<li>for 循环</li>
<li>for in:<code>for(let i in arr)</code> ，i 为索引</li>
<li>for of:<code>for(let item of arr)</code>，item 为每一项</li>
</ul>
</li>
<li>扩展运算符 […arrList] 展开数组 ，得到一个新的数组（console.log({…obj})）</li>
<li><code>filter</code><ul>
<li>filter 的回调必须返回一个<code>bool</code>值，<code>true</code>自动将这一项<code>返回</code>，否则<code>过滤</code>掉这一项</li>
<li>用<code>新数组</code>接收</li>
</ul>
</li>
<li><code>map</code><ul>
<li>每一项进行个性化操作后返回，生成<code>新数组</code>,改变<code>item</code>会音响原数组</li>
</ul>
</li>
<li><code>reduce</code>:对数组所以项进行汇总<ul>
<li>preVal 为<code>前一次return</code>的值，<code>第一次</code>默认为<code>参数二传入</code>的值，这里是 0<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> arr = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>];</span><br><span class="line"><span class="keyword">let</span> total = arr.<span class="title function_">reduce</span>(<span class="function">(<span class="params">preVal, item</span>) =&gt;</span> &#123;</span><br><span class="line">  <span class="keyword">return</span> preVal + item;</span><br><span class="line">&#125;, <span class="number">0</span>);</span><br></pre></td></tr></table></figure></li>
</ul>
</li>
<li><code>数组扁平化</code>并去重<ul>
<li><strong>…new Set([1,2,3,4,[4,5,6,7],7,8,9].flat(Infinity))</strong></li>
</ul>
</li>
</ul>
</li>
</ul>
<h2 id="字符串"><a href="#字符串" class="headerlink" title="字符串"></a>字符串</h2><p>模板字符串 <code>xxx$&#123;number&#125;xxx</code><br>字符串方法</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> str = <span class="string">&quot;123abc456&quot;</span>;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(str.<span class="title function_">includes</span>(<span class="string">&quot;abc&quot;</span>)); <span class="comment">// true</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(str.<span class="title function_">startsWith</span>(<span class="string">&quot;123&quot;</span>)); <span class="comment">// true</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(str.<span class="title function_">endsWith</span>(<span class="string">&quot;456&quot;</span>)); <span class="comment">// true</span></span><br></pre></td></tr></table></figure>

<h2 id="面向对象"><a href="#面向对象" class="headerlink" title="面向对象"></a>面向对象</h2><h3 id="对象"><a href="#对象" class="headerlink" title="对象"></a>对象</h3><ul>
<li>对象增强写法<ul>
<li>name:name -&gt; name</li>
<li>run:function(){} -&gt; run(){}</li>
<li>表达式 key<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> name = <span class="string">&#x27;lzo&#x27;</span></span><br><span class="line"><span class="keyword">let</span> obj = &#123;</span><br><span class="line">    name,</span><br><span class="line">    <span class="title function_">fun</span>(<span class="params"></span>) &#123; &#125;,</span><br><span class="line">    [name]: name</span><br><span class="line">&#125;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(obj) <span class="comment">// &#123;name: &#x27;lzo&#x27;, lzo: &#x27;lzo&#x27;, fun: ƒ&#125;</span></span><br></pre></td></tr></table></figure></li>
</ul>
</li>
<li>对象方法<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> user = &#123;</span><br><span class="line">    <span class="attr">name</span>: <span class="string">&quot;123&quot;</span>,</span><br><span class="line">    <span class="attr">age</span>: <span class="number">100</span></span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">let</span> objAssign = <span class="title class_">Object</span>.<span class="title function_">assign</span>(&#123;&#125;,user)  <span class="comment">// 合并 &#123;&#125; 和 user，实现浅拷贝</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> objCreate =  <span class="title class_">Object</span>.<span class="title function_">create</span>(user)<span class="comment">// 创建一个对象，并将对象 proto 指向参数对象</span></span><br><span class="line">                 <span class="title class_">Object</span>.<span class="title function_">create</span>(<span class="literal">null</span>)<span class="comment">// 创建一个没有原型的对象</span></span><br><span class="line"></span><br></pre></td></tr></table></figure></li>
</ul>
<h2 id="Promise"><a href="#Promise" class="headerlink" title="Promise"></a>Promise</h2><h2 id="generator"><a href="#generator" class="headerlink" title="generator"></a>generator</h2><h2 id="模块"><a href="#模块" class="headerlink" title="模块"></a>模块</h2><p>vue-vue2.md</p>
<h2 id="class"><a href="#class" class="headerlink" title="class"></a>class</h2><p>语法糖</p>
<h2 id="Map-and-Set"><a href="#Map-and-Set" class="headerlink" title="Map and Set"></a>Map and Set</h2><h3 id="Set"><a href="#Set" class="headerlink" title="Set"></a>Set</h3><blockquote>
<p>类似于数组的数据结构，成员值都是唯一且没有重复的值</p>
</blockquote>
<h2 id="常用功能"><a href="#常用功能" class="headerlink" title="常用功能"></a>常用功能</h2><p>结构赋值</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 数组</span></span><br><span class="line"><span class="keyword">let</span> arr = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>];</span><br><span class="line"><span class="keyword">let</span> [a, b, c = <span class="number">100</span>, d, e = <span class="number">150</span>] = arr;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(c); <span class="comment">// 10</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(e); <span class="comment">// 150</span></span><br><span class="line">c = <span class="number">10</span>;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(c); <span class="comment">// 10</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(arr); <span class="comment">// 还是 [1,2,3,4]</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 对象</span></span><br><span class="line"><span class="keyword">let</span> user = &#123;</span><br><span class="line">  <span class="attr">name</span>: <span class="string">&quot;123&quot;</span>,</span><br><span class="line">  <span class="attr">age</span>: <span class="number">100</span>,</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">let</span> &#123; name, <span class="attr">age</span>: age2 &#125; = user;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(age2); <span class="comment">// 100</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(name); <span class="comment">// 123</span></span><br><span class="line">name = <span class="number">456</span>;</span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(name); <span class="comment">// 456</span></span><br><span class="line"><span class="variable language_">console</span>.<span class="title function_">log</span>(user); <span class="comment">// 不变</span></span><br></pre></td></tr></table></figure>
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class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link"><span class="toc-number">2.1.</span> <span class="toc-text">??</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#-1"><span class="toc-number">2.2.</span> <span class="toc-text"></span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%87%BD%E6%95%B0"><span class="toc-number">3.</span> <span class="toc-text">函数</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%AE%AD%E5%A4%B4%E5%87%BD%E6%95%B0"><span class="toc-number">3.1.</span> <span class="toc-text">箭头函数</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%95%B0%E7%BB%84-json"><span class="toc-number">4.</span> <span class="toc-text">数组&#x2F;json</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%AD%97%E7%AC%A6%E4%B8%B2"><span class="toc-number">5.</span> <span class="toc-text">字符串</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%9D%A2%E5%90%91%E5%AF%B9%E8%B1%A1"><span class="toc-number">6.</span> <span class="toc-text">面向对象</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%AF%B9%E8%B1%A1"><span class="toc-number">6.1.</span> <span class="toc-text">对象</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#Promise"><span class="toc-number">7.</span> <span class="toc-text">Promise</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#generator"><span class="toc-number">8.</span> <span class="toc-text">generator</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%A8%A1%E5%9D%97"><span class="toc-number">9.</span> <span class="toc-text">模块</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#class"><span class="toc-number">10.</span> <span class="toc-text">class</span></a></li><li class="toc-item toc-level-2"><a 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